Modeling of Bradley Tracked Vehicle Steering and Fuel Consumption with a Detailed Kinematic Model of the HMPT500-3 Transmission and Simplified Vehicle Dynamics

The HMPT500-3 is a split torque path hydrostatic / mechanical CVT used in the Bradley Fighting Vehicle. A previous paper detailed a linear algebraic approach to model forward operation of the HMPT500-3 without steering using a reduced equation set. This model was expanded to the full equation set to allow transmission operation with steering. The equations showed that opposite to a typical automotive “open” differential, the HMPT500-3 enforces a speed difference between the sprockets for steering, but does not have an inherent torque bias. The typical regenerative steering torque from the decelerated inside track must be provided by interfacing with a vehicle model. A simplified 2-D planar dynamics model of the Bradley was developed to explore vehicle performance and fuel consumption with steering. The integrated model showed that fuel consumption during minimum radius turns can double that of straight-ahead operation at the same speed. Commercial vehicle performance codes lack tracked steering models, so test courses are run as simple gradient vs. distance profiles which would result in underestimating fuel consumption. The Munson Fuel Loop course was selected to attempt to quantify the contribution of steering to total fuel consumption. Fuel consumption calculations with steering were found to be 9% to 14% higher than those without steering depending on lap speed, and compared more favorably with Bradley test data.